I've also been attempting to look into this and came up with an almost identical patch, which is promising:
https://bitbucket.org/tlynn/issue1859/diff/textwrap.py?diff2=041c9deb90a2&diff1=f2c093077fbf

I missed the wordsep_simple_re though.

Testing it is the hard part.  I've got a few examples that could become tests in that repository, but it's far from conclusive.

One corner case I found is trailing whitespace becoming a blank line:

>>> from textwrap import TextWrapper
>>> T = TextWrapper(replace_whitespace=False, drop_whitespace=False, width=9)
>>> T.wrap('x'*9 + ' \nfoo')
['xxxxxxxxx', ' ', 'foo']

I think it's fine.  drop_whitespace=True removes the blank line, and those who really want drop_whitespace=False can remove the blank lines easily.